Vanadium (IV) dioxide, VO2, has attracted significant research interest owing to its orders-of-magnitude first-order phase transition between insulating and metallic states that occurs at ca. 67° C. in the bulk. The metal-insulator transition temperature, TMIT, is uncommonly close to room temperature, and this fortunate circumstance has inspired considerable interest in device architectures that can take advantage of the abrupt switching of electrical and optical properties accompanying this phase transition. A partial list of proposed device architectures that have been experimentally realized (to varying extents) includes Mott field transistors, spectrally selective thermochromic glazings for “smart window” applications, frequency-agile metamaterials for electromagnetic cloaking, periodic oscillators, memory devices based on two-terminal device configurations (mimicking neuromorphic circuits), and strain sensors. In addition, VO2 has long served as an accessible system for fundamental explorations of strongly correlated behavior in materials. The occurrence of a structural phase transformation between monoclinic and tetragonal phases in close proximity to the electronic transition temperature has led to contrasting views regarding the relative importance of electron-electron and electron-phonon interactions in VO2. However, it must be noted that the electronic and structural phase transitions can be separated for both thermally induced and photoactivated processes in this system. Scaling VO2 to finite sizes allows for more robust accommodation of the strain generated during the structural phase transformation (in other words, enables protracted thermal cycling without cracking) and additionally allows for modification of the phase diagram to suppress the transition temperature and stabilize metastable phases.
From a synthetic perspective, VO2 is a challenging target since it is only stable within a narrow sliver of the V—O binary phase diagram where it resides in immediate proximity to oxygen-deficient Magneli and Wadsley-type phases wherein extended defects such as crystallographic shear planes facilitate accommodation of periodic arrays of oxygen vacancies. The magnitude of the metal-insulator transition (for both optical and electrical properties) and TMIT thus vary sensitively as a function of oxygen stoichiometry in VO2. VO2 nano- or micro-structures derived from solution-phase methods are often plagued by broadened and relatively diminished metal-insulator transitions due to imperfect control of stoichiometry.
Optimal control of the stoichiometry of VO2 and thereby materials exhibiting the most pronounced (above three orders of magnitude) phase transitions have been realized thus far primarily by physical vapor deposition methods such as high-temperature vapor-solid deposition, molecular beam epitaxy, pulsed laser deposition, and sputtering. In these methods, the structure can be dictated based on epitaxial homology with an underlying lattice-matched substrate, whereas the stoichiometry can be precisely defined by tuning the background pressure, precursor concentrations, and annealing conditions. Such variables tend to be more difficult to control in solution-phase syntheses and indeed homogeneous nucleation tends to favor stabilization of a metastable VO2(B) structure with the exception of reactions performed under high pressures.
Obtaining free-standing nanowires further allows for deployment of surface functionalization and colloidal chemistry approaches thereby enabling tuning of properties, incorporation within different matrices, and permitting the rational design of multicomponent nanocomposites. Sol-gel and hydrothermal approaches have previously been used to prepare VO2 nanowires; the incorporation of W and Mo dopants appears to facilitate stabilization of the rutile (and upon cooling to room temperature, the M1 monoclinic) phase of VO2.